The present invention relates to an offset drift correction in a color film inspection apparatus.
A color film inspection apparatus is used to display a color image or images of a color original on a CRT screen based on which color prints to be finished are visually evaluated prior to photo-printing. Such a color film inspection apparatus is generally provided with the same automatic correction system for color density and color balance as is provided in automatic printing apparatus by which finished color prints are stimulated and displayed on a color monitor such as a color CRT. From a visual inspection of the color image displayed on the color CRT, color originals improper in color density, color balance, and/or sharpness are checked. In order to determine the correct amount of color density and color balance, each color original is displayed on the color CRT through color and density simulation. By using the color film inspection apparatus, a precise color film inspection may be performed even by unskilled inspectors, resulting in a high quality of color prints.
An offset drift may occur in such an inspection apparatus due to electric circuits dependent an temperature changes, light and time (its adjusting elements change with time). If in fact the offset drift occurs, the images displayed on the CRT screen through a simulation will change in color balance or density, resulting in improper inspections. For this reason, an offset drift correction is conducted in order to make color prints with a high image quality.
One such offset drift correction method is disclosed in, for example, Japanese Patent Unexamined Publication No. 61-138,942. The offset drift correction taught by the above-mentioned Japanese Patent Unexamined Publication is a method to shift logarthmic-transforming curves for three colors, namely red, green and blue, written in a look-up table memory along the X-axis (input).
In such an offset drift correction method, standardized logarithmic-transforming curves for red, green and blue are provided to effect an offset drift correction by color. However, this offset drift correction has a problem that an error in color balance occurs. For example, a white original (which appears as a black image on a color negative) is displayed as a different color (not white) image on a color CRT, so the white image on a color original cannot be faithfully reproduced as the same white image.